Flexible interface structures for electronic devices

ABSTRACT

A flexible film interface includes a flexible film; flexible material attached to a portion of the flexible film; surface metallization on the flexible material, the flexible film having at least one via extending therethrough to the surface metallization; and a floating pad structure including floating pad metallization patterned over the flexible material and the surface metallization, a first portion of the floating pad metallization forming a central pad and a second portion of the floating pad metallization forming at least one extension from the central pad and extending into the at least one via.

The invention was made with Government support under contract numberF29601-92-C-0137 awarded by the United States Air Force.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation in part of U.S. application Ser. No.08/781,972, filed Dec. 23, 1996, which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

Ball grid array (BGA) technology provides a high density ofinterconnections per unit area, but mismatches of coefficients ofthermal expansion (CTEs) occur when ceramic or polymer BGA substratesand printed circuit boards are joined and often result in cracked solderjoints, especially as the size of the substrates and temperature rangesare increased. In column grid array (CGA) techniques and other BGAtechniques, a eutectic solder is applied to printed circuit board andmulti-chip module array pads and the resulting joint is soldered to ahigher temperature solder column or ball which does not melt. Both BGAand CGA structures can be inflexible and vulnerable to damage. Forvarious types of BGA and CGA, increases in reliability are attempted byelaborate under-filling of the structures with polymer glues toreinforce the interfaces and reduce the effects of the CTE mismatch onthe solder joints. The polymer glues, however, impair repairabilitybecause of the difficulty in removing the glues after hardening.Furthermore, these types of structures require two separate soldersteps, are more expensive than conventional solder structures, andrequire more vertical space due to increased height of the joints.

One conventional micro ball grid array interface technique for attachinga semiconductor circuit chip directly to a substrate is to use a seriesof solder bumps clustered at the center of the chip to constrain thearea over which stresses between differing coefficients of thermalexpansion occur. In this embodiment, chips have their pads reconfiguredand solder micro bumps are applied over the reconfigured pads. In oneembodiment, ball grid array processes are used with the temperaturerange being constrained during device operation to 30° C. to 70° C. inan effort to avoid CTE stress effects. In another ball grid arrayinterface technique, the area where the chip faces the printed circuitboard or substrate is not used for direct interconnection. Instead,metallization is routed from the chip to adjacent support structureswhich then have solder ball connections. This technique can create sizeand pin count limitations as well as electrical parasitic effects.

Aforementioned U.S. application Ser. No. 08/781,972, describes aninterface including a surface having an electrically conductive pad; acompliant coating over the surface having a via extending to the pad;metallization patterned over the compliant coating and extending intothe via; a low modulus dielectric interface layer overlying thecompliant coating and having an interface via extending to themetallization; and a floating pad structure including floating padmetallization patterned over the dielectric interface layer with a firstportion forming a central pad and a second portion forming an extensionfrom the central pad extending into the interface via. The "floatingpad" structure is used to increase reliability by providing stress andthermal accommodation of the two materials and permitting movement ofthe floating pad independent of the base pad. The extension providesstress relief for different coefficients of thermal expansion. Thefloating pad interface structures can include a single pad and extensionor several extensions in situations wherein a single extension is notsufficient for extreme thermal stress/strain situations. The resultingstructure accommodates thermal and material stresses without submittingthe via interconnect areas to forces that can crack vias or breakconnections at the pads. The floating pads permit movement independentof a base surface underlying the pads while providing electricalinterconnections through selected materials that are specificallypatterned to provide low forces at the via areas and thus accommodatedifferential thermal stresses which may be caused by large CTEdifferences.

SUMMARY OF THE INVENTION

It would be desirable to additionally have a more flexible interfacestructure for electronic devices that does not require an underlyingbase surface and that can be used for relieving stress from structuressuch as multi-chip modules (MCMs), wafers, individual dies or chips,microelectromechanical structures (MEMs), printed circuit boards, andsurface mount technologies which may be caused from coefficient ofthermal expansion mismatches with connections such as those formed byball grid arrays, micro ball grid arrays, column grid arrays, flipchips, solder joints, or tape automated bonding connections.

In one embodiment of the present invention, a flexible film interfaceincludes a flexible film; flexible material attached to a portion of theflexible film; surface metallization on the flexible material, theflexible film having at least one via extending therethrough to thesurface metallization; and a floating pad structure including floatingpad metallization patterned over the flexible material and the surfacemetallization, a first portion of the floating pad metallization forminga central pad and a second portion of the floating pad metallizationforming at least one extension from the central pad and extending intothe at least one via.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings, where like numerals represent like components, in which:

FIG. 1 is a sectional side view of a flexible film having attachedthereto flexible material with surface metallization.

FIG. 2 is a view similar to that of FIG. 1 further including floatingpad structures overlying the flexible film.

FIG. 3 is a top view of one of the patterned floating pad structuresoverlying a portion of the surface metallization.

FIG. 4 is a view similar to that of FIG. 2 further showing an optionalsolder mask extending over the flexible film.

FIG. 5 is a view similar to that of FIG. 4 further showing attachment ofthe surface metallization to base pads through a solder ball and anepoxy ball.

FIG. 6 is a view similar to that of FIG. 5 further showing attachment ofthe floating pad structures to conductive pads through a ball gridarray.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a sectional side view of a flexible film 10 having attachedthereto flexible material 12 and 14 with surface metallization 16. Theflexible film 10 preferably comprises a flexible polymer such as, forexample, KAPTON™ polyimide (KAPTON is a trademark of DuPont Co.). In oneembodiment, the flexible film has a thickness ranging from about 7.5micrometers (0.3 mils) to about 125 micrometers (5 mils).

The flexible material 12 and 14 preferably comprises a flexible lowmodulus material, such as SPI/epoxy, other flexible epoxies, siliconerubber materials, MULTIPOSIT™ XP-9500 thermoset epoxy (MULTIPOSIT is atrademark of Shipley Company Inc., Marlborough, Mass.), TEFLON™polytetrafluoroethylene (TEFLON is a trademark of DuPont Co.), porouspolytetrafluoroethylene, or other polymers that have a low modulus orhave been modified to obtain reduced modulus, having a thickness rangingfrom about 0.5 millimeters (20 mils) to about 1.250 millimeters (50mils) depending on the application and the net differential stress to beaccommodated. The flexible material provides a soft pliable interface topermit a later applied floating pad to move easily as CTE mismatches areaccommodated.

The flexible material can be applied by spin coating, spray coating,film photodeposition, or roll coating, for example. The material can bepatterned by photo-patterning techniques and/or masking, for example. Asshown in FIG. 1, the flexible material can have either a uniformthickness (as shown by flexible material 12) or a varying thickness (asshown by flexible material 14). The tapered edges of flexible material14 can be useful for reducing stress on surface metallization 16.

Surface metallization 16 can be formed by sputtering and/or plating, forexample, either through a mask or over the entire surface followed bypatterning with a standard photoresist and etch process. Eichelberger etal., U.S. Pat. No. 4,835,704, issued May 30, 1989, describes a usefuladaptive lithography system for patterning the metallization, forexample. The surface metallization in one embodiment comprises a thinadhesion layer of about 1000 Å to about 2000 Å sputtered titanium,coated by a thin layer of about 3000 Å to about 5000 Å sputtered copper,coated by a layer of electroplated copper to a thickness of about threemicrons to about ten microns, for example. The appropriate material ofthe surface metallization will vary depending on the material it isexpected to contact and on the environment, such as a high temperatureenvironment or an oxidizing environment for example, in which theelectronic device will be used. If the surface metallization will be incontact with solder, preferably nickel and gold (with a thickness about1000 Å to about 2000 Å) are applied over the electroplated copper. Ifthe surface metallization will be in contact with a conductive polymer,titanium (with a thickness of about 1000 Å to about 2000 Å) is a usefulmaterial to apply over the copper.

FIG. 2 is a view similar to that of FIG. 1 further including floatingpad structures 30 overlying the flexible film 10 to form a flexibleinterface structure 1. Vias 24 are formed in flexible film 10 and extendto surface metallization 16 by any appropriate method. In oneembodiment, as described in Eichelberger et al., U.S. Pat. No.4,894,115, issued Jan. 16, 1990, the flexible film can be scannedrepeatedly with a high energy continuous wave laser to create via holesof desired size and shape. Other appropriate methods include, forexample, photopatterning photopatternable polyimides and using anexcimer laser with a mask (not shown).

In a preferred embodiment, as shown in FIG. 2 surface metallization 16has one portion 20 covering the flexible material and another portion 21which extends to contact flexible film 10. In this embodiment, it isuseful to have vias 24 extend to contact portion 21 of the surfacemetallization. Although this embodiment is preferred, the presentinvention can be practiced if the vias extend to portions of the surfacemetallization that are in contact with the flexible material.

FIG. 3 is a top view of one of patterned floating pad structures 30overlying a portion 20 of surface metallization 16. Metallization forfloating pad structures 30 can be applied and patterned by techniquessimilar to those discussed with respect to surface metallization 16 ofFIG. 1. In FIGS. 2 and 3, the floating pad structures include a centralpad 26 having patterned extensions 28 extending through vias 24 tosurface metallization 16.

The size of central pads 26 will vary according to the specific planneduse of the floating pad structure. For example, if a solder ball orsolder bump will be attached directly to the central pad, the centralpad must be large enough to accommodate the attachment.

As discussed in aforementioned U.S. application Ser. No. 08/781,972, thethickness of metallization for the floating pad structures can beuniform or variable (with the central pad being thicker than theextensions). In one embodiment, the extensions 28 have a thicknessranging from about 2 microns to about 8 microns, and the central pad 26has a thickness ranging from about 2 microns to about 20 microns. Thisembodiment is useful because thin extensions are more flexible thanthicker extensions whereas the central pad is preferably sufficientlythick for attachment to another electrically conductive surface. Asdiscussed and shown in aforementioned U.S. application Ser. No.08/781,972, any number of extensions (one or more) can be used, and theextensions can have any shape. Examples include straight extensions,serpentine shaped extensions, spiral extensions, saw-tooth extensions,bent extensions, pin-wheel shaped extensions, and extensions whichextend to a ring which may in turn have additional extensions extendingto another ring (not shown).

As further discussed and shown in aforementioned of U.S. applicationSer. No. 08/781,972, a dip can intentionally be formed in the flexiblefilm for reducing mechanical stress on extensions. The can be formedprior to, during, or after application of the flexible material byetching or by heat pressing, for example. If a dip is formed in theflexible film, extensions 28 will then have dips for stress relief.Additional metallization can be applied to the central pad if desired.Although not shown, the embodiments of FIGS. 2 and 4-6 are expected tohave some natural (and beneficial) dips resulting from the applicationof the metallization of the floating pad structures even if dips are notintentionally formed.

FIG. 4 is a view similar to that of FIG. 2 further showing an optionalsolder mask 32 extending over the flexible film. This mask is useful ifcentral pad 26 is to be soldered to another conductive surface or padfor preventing solder leaching or solder run off onto extensions 28. Thesolder mask may comprise any material which is capable of maskingsolder. Examples include photopatternable epoxies that can be UV exposedand heat cured.

FIG. 5 is a view similar to that of FIG. 4 further showing attachment ofsurface metallization 16 to base pads 112 through a conductive material116. The conductive material may comprise a solder ball or a conductivepolymer, for example. A base member 110 underlying base pads 112 maycomprise a semiconductor wafer that has not yet been cut into segmentedindividual chips, a chip which has been segmented from a wafer, apassive component, a printed circuit (PC) board, a multi-chip module(MCM), a flexible interconnect layer structure such as described in Coleet al., U.S. Pat. No. 5,527,741, Jun. 18, 1996, or a substrate or waferwhich may include photonic structures, liquid crystal structures, ormicroelectromechanical structures (MEMS) such as described in commonlyassigned Ghezzo et al., U.S. Pat. No. 5,454,904, issued Oct. 3, 1995,for example. Base pads 112 may comprise pads or metallization on any ofthe above-discussed base members.

The flexible interface structure can be fabricated separately from thebase member and later attached by the solder or by a conductive polymersuch as a silver or a gold epoxy, for example. If solder is used, asolder mask 114 can be applied over the base member. If the base membercomprises an MCM, a high temperature lead solder (such as 10 parts tinand 90 parts lead) is useful. If the base member comprises a PC board, alower temperature solder (such as 37 parts tin and 63 parts lead) can beused.

FIG. 6 is a view similar to that of FIG. 5 further showing attachment ofthe floating pad structures 30 to mount pads 140 through a ball gridarray 136 of a mount member 142. BGA solder balls 136 can be applied andsolder can be reflowed to the floating pad structure. The mount membermay comprise structures and devices similar to those of the base member.Examples of particularly useful mount members include flip chips,surface mount devices, ceramic filters, resistor assemblies, ceramiccapacitors, MEMS, and large processor die that are fitted with solderpad (micro BGA) structures. If desired, surface metallization 16 offlexible interface structure can be patterned to fan out of micro BGA toa full sized BGA for positioning or for mechanical reasons. Many devicescan be simultaneously attached to base member 110 using this device.

FIG. 6 further illustrates the removal of at least a portion of flexiblefilm 10 and at least a portion of flexible material 12 and 14. If theflexible material is to be removed and if (as preferred) the surfacemetallization 16 surrounds the flexible material, at least a portion 128of the flexible film (as well as any overlying solder mask, ifapplicable) will need to be removed from the surface adjacent theflexible material prior to removal of any flexible material. Preferably,no portion of the flexible film is removed from the areas under thecentral pad or the extensions.

In this embodiment, the material of the flexible film is preferablychosen to be a material that can be removed by a laser ablation orpolymer ashing, for example, and the material of the flexible materialis chosen to be a material that can be removed by a sublimation processor a solvent, for example, without interfering with the other materialsto leave openings 150. Such partial removal of the flexible film and/orpartial or total removal of the flexible material permits increasedfreedom of movement.

If desired, multiple levels of floating pads (not shown) can be used.Such additional layers are useful for providing greater stressaccommodation when a single floating pad layer is not sufficient forextreme cases of thermal stress or strain. Multiple layer interfaceembodiments create additional thermal-mechanical isolation because thefloating pad structures are farther away and provide a compound leverand thus more degrees of physical freedom.

While only certain preferred features of the invention have beenillustrated and described herein, many modifications and changes willoccur to those skilled in the art. It is, therefore, to be understoodthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

What is claimed is:
 1. A flexible film interface comprising:a flexiblefilm; flexible material attached to the flexible film; surfacemetallization on the flexible material; the flexible film having atleast one via extending therethrough to the surface metallization; afloating pad structure comprising floating pad metallization patternedover the flexible film opposite the flexible material and the surfacemetallization, a first portion of the floating pad metallization forminga central pad and a second portion of the floating pad metallizationforming at least one extension from the central pad and extending intothe at least one via, the floating pad structure capable ofaccommodating thermal and material stresses and permitting movements ofthe central pad independent of the surface metallization.
 2. Theinterface of claim 1 wherein the floating pad metallization includes atleast two extensions from the central pad and wherein the flexible filmhas at least two vias therein, each extension extending into arespective via.
 3. The interface of claim 2 wherein portions of thesurface metallization are in contact with the flexible film and comprisefilm portions of the surface metallization, and wherein the at least twovias extend to the film portions of the surface metallization.
 4. Theinterface of claim 3 wherein the flexible material comprises a lowmodulus polymer.
 5. The interface of claim 4 wherein the flexiblematerial has tapered edges.
 6. The interface of claim 3 furtherincluding a solder mask extending over the at least two extensions. 7.The interface of claim 3 further including a conductive materialattached to the surface metallization, the conductive materialcomprising a solder ball or a conductive polymer.
 8. An electronicpackage comprising:(a) a flexible film interface including:a flexiblefilm; flexible material attached to the flexible film; surfacemetallization on the flexible material and on a same side of theflexible film as the flexible material, the flexible film having atleast two vias extending therethrough to the surface metallizationsituated on the flexible film; and a floating pad structure comprisingfloating pad metallization patterned over the flexible film opposite theflexible material and the surface metallization, a first portion of thefloating pad metallization forming a central pad and a second portion ofthe floating pad metallization forming at least two extensions from thecentral pad and extending into the at least two vias, the floating padstructure capable of accommodating thermal and material stresses andpermitting movements of the central pad independent of the surfacemetallization; (b) a base member having a at least one conductive basepad; and (c) a conductive material coupling the surface metallizationand the at least one base pad.
 9. The package of claim 8 wherein theconductive material is a solder ball or a conductive polymer.
 10. Thepackage of claim 9 wherein the base member is a printed circuit board, amulti-chip module, a semiconductor wafer, a chip, a passive component,or a flexible interconnect layer structure.
 11. The package of claim 9further including a mount member having at least one conductive mountpad and a second conductive material coupling the central pad of thefloating pad structure and the at least one mount pad.
 12. The packageof claim 11 wherein the mount member comprises a flip chip or a ballgrid array.
 13. A flexible film interface comprising:a flexible film;surface metallization attached to a portion of the flexible film; theflexible film having at least one via extending therethrough to thesurface metallization; a floating pad structure comprising floating padmetallization patterned over the flexible film opposite the surfacemetallization, a first portion of the floating pad metallization forminga central pad and a second portion of the floating pad metallizationforming at least one extension from the central pad and extending intothe at least one via, the floating pad structure capable ofaccommodating thermal and material stresses and permitting movements ofthe central pad independent of the surface metallization.